The present invention is directed to austenitic manganese steel castings having improved wear resistance resulting from grain refinement due to the additions of vanadium, titanium and nitrogen, and methods of producing this steel in applications such as, for example, casting wear liners for cone and jaw crushers, hammers for scrap shredders, frogs and switches for railway tracks and other castings required to possess gouging, abrasion and impact resistance.
Austenitic manganese steels having a wide range of applications are well known. Such steels include alloying additions of manganese (Mn) in amounts of 5-25% by weight and carbon (C) content in the range of about 0.7-2.0% by weight. The most characteristic type is the austenitic Mn-steel containing 12-14% Mn and 1.2-1.4% C, which was invented in 1882 by Robert Hadfield and to this day often is referred to as Hadfield steel. These steels combine high toughness with ductility and high work-hardenability which makes them a material of choice for wear components of machinery and equipment used in mining, quarrying, earthmoving, dredging and the railroads, to name the most significant fields of application.
One example of such an austenitic manganese steel is set forth in U.S. Pat. Nos. 4,512,804 and 4,531,974 to Kos. These patents are directed to a work-hardenable austenitic manganese steel having carbon to manganese ratios between 1:4 and 1:14 and microalloyed with 0-0.20% by weight of titanium (Ti), 0-0.05% by weight zirconium (Zr) and 0-0.05% by weight vanadium (V), provided that the sum of Ti+Zr is in the range of 0.003-0.05 weight percent. These alloying elements are added to refine the grain size of the casting, which grain size can be further refined by the addition of small amounts of boron (B). Alternatively, Ti in the range of 0.01-0.025% or Ti+Zr+V in the range of 0.002 to 0.05 when microalloyed with the austenitic steel produced castings having refined grain size. These alloying elements, when added to the casting ladle after a deoxidation process, have produced a manganese steel with exceptional toughness. The alloys set forth in these patents obtain their grain refinement by the use of microalloying additions of zirconium and titanium, while vanadium is an optional element.
Another alloy is set forth in Canadian Patent Application No. CA 1221560 to Kos. This alloy is similar to the alloys set forth above, but allows up to 0.20% titanium, in addition to optional amounts of vanadium and zirconium. The Canadian application broadly identifies the compositions set forth in the earlier U.S. patents, but fails to appreciate the benefits that can be achieved by the interaction of several key elements when closely controlled within relatively tight limits and when processed to maximize their effect on the product.
While each of the above-described alloys represents advancement in the art resulting from the careful control of grain size in large castings, further advancements are sought to improve efficiency and reduce overall costs by improving the wear resistance of these castings by continuing the control of grain size, and by making further improvements.
What is needed is an alloy that can extend the mean life of wear components subjected to gouging, abrasion and/or impact like the one that occurs in rock crushers, mining power shovels, scrap shredders, frogs and switches used in railroad crossings and others.
The present invention is an austenitic manganese steel microalloyed with nitrogen, vanadium and titanium, used for castings such as mantles, bowls and jaws used as wear components in the mining and aggregate industries, hammers used in scrap shredders, buckets and track shoes used in mining power shovels, frogs and switches used in railroad crossings. The compositions made in accordance with the present invention exhibit a fine grain size having carbonitride precipitates, and titanium-containing carbonitride precipitates, that result in castings having a wear life 20-70% longer than prior art castings.
The austenitic manganese steel of the present invention is comprised, in weight percentages, of the following: about 11.0% to 24.0% manganese, about 1.0% to 1.4% carbon, up to about 1% silicon, up to about 1.9% chromium, up to about 0.25% nickel, up to about 1.0% molybdenum, up to about 0.2% aluminum, up to about 0.25% copper, phosphorus and sulfur present as impurities in amounts of about 0.07% max. and about 0.06% max., respectively, microalloying additions of titanium in the amounts of about 0.020-0.70%, optionally, microalloying additions of niobium in amounts from about 0.020-0.70%, microalloying additions of vanadium in amounts from about 0.020-0.70%, nitrogen in amounts from about 100 to 1000 ppm, and such that the total amount of the microalloying additions of titanium+niobium+vanadium+nitrogen is no less than about 0.05% and no greater than about 0.22%, the ratio of carbon to microalloying additions being in the range of about 10:1-25:1, and the balance of the alloy being essentially iron, the alloy being characterized by a substantial absence of zirconium and the presence of titanium-containing precipitates, for example, titanium carbonitride precipitates. The alloy otherwise conforms to ASTM Standard A128/A128M-93.
While the alloy of the present invention may contain small amounts of zirconium, the amounts of zirconium present must be, on an atomic level, less than the amount of nitrogen.
Small deviations of the chemistry from the relatively tight ranges set forth above result in a failure to achieve the desired grain size with a subsequent loss of the beneficial effects of improved wear resistance exhibited by the alloy of the present invention.
The alloy of the present invention is very sensitive to processing. The alloy of the present invention is melted in an electric arc furnace or an induction furnace. In order to obtain the beneficial effects of the microalloying elements, it is necessary to deoxidize the molten metal prior to microalloying.
Conditions in the molten steel must promote the formation of the carbonitride precipitates, including, titanium carbonitride precipitates. It is known that failure to properly deoxidize the molten metal results in a loss of titanium as TiO2. Furthermore, vanadium can be added to the furnace or ladle, although titanium and carbide-forming elements should be added to the molten metal as it is transferred from the furnace to a pouring ladle in order to obtain proper distribution of these elements in the molten bath. In practice vanadium, titanium, optional niobium, and any other carbide-forming elements, are added to the molten metal during the metal transfer from the furnace to the pouring ladle. Alternatively, the proper distribution can be achieved by agitation of the molten metal in the pouring ladle. The pouring temperature of the molten metal must be carefully controlled in accordance with good foundry practice. Castings made of an alloy processed in accordance with the present invention has a refined grain size of #1 or finer as determined in accordance with ASTM standard E-112 in test bars having a 4xe2x80x3 cross-section. As used herein, all references to grain size, and specifically to ASTM E-112#1 or finer grain size, is with reference to the average grain size measured in a test bar having a 4xe2x80x3 cross-section. As recognized by those skilled in the art, different cross sections can be expected to display different grain size results. Castings made in accordance with the present invention are expected to display an average grain size that is finer than castings not made in accordance with the present invention.
An advantage of castings having compositions and processed in accordance with the present invention is that they have markedly improved wear properties. Thus, the castings used in applications in which wear is a consideration, such as mantels, bowl liners, jaws, hammers, dipper buckets, frogs and other similar parts, have a decided advantage when resistance to wear is increased. The major benefits include longer mean life between replacements. This in turn means lower operating costs, for an increase in mean life between replacements meaning fewer replacement parts, lower labor costs as less labor time is spent replacing worn parts and less down time. These benefits are significant even if the cost of the casting having improved resistance to wear is slightly higher than castings not exhibiting such improvements.
Another advantage of the present invention is that the alloy of the present invention can be made using existing equipment, provided that the processing controls required by the present invention are implemented.
Still another advantage of the present invention is that increased wear life provided by the castings of the present invention will ultimately result in a conservation of resources. Since the life of each casting is longer, less energy is expended to produce and transport fewer castings, and fewer pollutants are released to the atmosphere.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.